1. Field of the Invention
The subject invention relates to liquid crystal displays and more particularly to driver circuitry fabricated on the same substrate as a liquid crystal display and employing selective feedback to compensate the video output signal of the driver circuitry to accommodate for variations in device threshold of the driver circuitry devices.
2. Description of Related Art
Liquid crystal matrix displays are known in general, for example, as disclosed in U.S. Pat. No. 3,862,360, assigned to Hughes Aircraft Company. In such displays, an analog signal, such as a video signal, is used to control the gray level of each display cell or "pixel." This analog or video signal is applied on a plurality of column supply buses or "display lines" and is selectively gated at the appropriate time to each display cell of the display by gate signals applied to a plurality of row or gate supply buses. Such displays typically employ one line driver per display line, sometimes referred to as "direct drive." The line drivers are typically not located on the substrate carrying the display cells, but rather are arrayed linearly adjacent one edge of the display substrate along a distance of several inches. As shown in the '360 patent, such line drivers may constitute the outputs of a dual serial to parallel video converter positioned adjacent the display substrate in order to properly interface a standard raster format video signal to the liquid crystal display.
Use of one off-substrate driver to drive several lines of a display, i.e. multiplexing of line driver inputs by a small factor, has also been attempted in the prior art. The number of line driver inputs which can be multiplexed using conventional on-display circuitry is restricted by the slow response speed of a display line. This slow response speed is caused by the relatively large capacitance and resistance of the display line.
Since the display line responds so slowly, it has appeared impossible to use one output of an off-substrate (also referred to as "off-panel" or "off-chip") driver to control large numbers of lines on the display, unless some integrated line driver circuitry is provided on the display substrate for each line of the display. However, displays using on-chip line drivers suffer from either severe process control requirements or non-uniform gray levels. Such problems arise because of the variation in the voltage required to turn on the transistor elements of the amplifiers of on-chip driver circuitry. As a result of processing variations across the several inches of distance along the edge of a display, the turn-on or "threshold" voltage of such transistors varies, resulting in non-uniform amplification and hence non-uniform gray levels, i.e. a distorted image. Only imposition of severe, impractical process control requirements can minimize the threshold voltage variation between adjacent transistor devices.
With the continuing need to reduce interconnection complexity as higher resolution, higher density liquid crystal displays evolve, it would be highly desirable to have the capability to control four or more lines of a display from a single driver output. However, because of the aforementioned problems, prior art approaches are believed to lack such capability.